yet another rafter tie vs ridge beam question

[SLTA]

Hi folks,

Reading the IRC 2009, it sounds to me like a ridge beam, designed to carry vertical load and spanning to posts etc, is NOT required if rafter ties are used - only a non-structural beam is required to facilitate rafter connections. Is this correct? In other words, the vertical loads are then assumed to be taken at the walls, not at the ridge (connection details aside).

Interesting how little information there is out there of the basics of ridge beams, rafter ties, etc. And yes, I read through the archives before writing this.

thanks,
Linnea

 

[msquared48]

I would say it depends.

The rafter ties spacing is critical here. If 2 feet as in normal truss spacing, yes. But not at 8 to 12 feet as some tied rafter systems are. There, ridge beams WILL be required, regardless of any code stipulation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SLTA]

thanks, Mike. I'll be using the rafter ties as the ceiling for the raised part, so they'll be at 16" oc like the rafters.

 

[TTUengr51]

Be mindful of how high above the wall plate the rafter ties are placed for raised ceilings, as raised placement will induce additional flexural load to the rafters and you could potentially start pushing the walls out with thrust forces generated.

Nick Deal, PE, SE
Michael Brady Inc.

http://www.michaelbradyinc.com

 

[BAretired]

I believe your original premise is correct if all rafters are tied, regardless of rafter spacing.

If there are any untied rafters, they would require a beam at the ridge.

BA

 

[msquared48]

BA:

Then you will be getting into the use of purlins transverse to the collar ties, which at the longsr spans, would actually be more like trusses. To construct such a system without the purlins depends on the ability of the plywood, or specific roof diaphragm material, to span the gap. A four foot span would be the limit, depending on the snow load, without purlins.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Mike:

What is wrong with tied rafters at 8' centers and steel or timber deck spanning the gap?

BA

 

[msquared48]

If it will span the 8 feet, nothing.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Also beware that raising the rafter ties may start to cause the rafters to creep/bow inward after some amount of time due to the tension force of the ties. Also these connections can be difficult and the rafter bending stress can get quite high. It seems alot of people want to do this and I don't think there is any reason not to but the architects are always surprised at what sizes it requires.

EIT

 

[BAretired]

A tied arch is a statically determinate structure, so there can be no argument as to the bending stresses involved. Surprising architects is nothing new.

BA